1. Field of the Invention
The present invention relates to a collimating lens. This application is based on Japanese Patent Application No. 2002-172969, the content of which is incorporated herein.
2. Description of Related Art
In recent years the internet has become progressively more widely used and the volume of communication data has progressively increased. Consequently, even greater increases in speed and volume are being sought in optical communication systems. As a result, signal multiplexing is being attempted. An example of this is the dense wavelength division multiplexing (DWDM) communication system. In DWDM signals are placed on 200 to 1000 types of wavelength enabling signal multiplexing to be performed. Because of this, DWDM is being studied as an optical communication system providing increased speed and volume.
Conventionally, in this type of optical communication system, a light source that is capable of outputting light in a plurality of wavelengths (such as a semiconductor laser) is used. A technology is also employed in which optical signals of the different wavelengths being output from the light source are combined in a transmission device such as an optical fiber using an optical element. For example, divergent light whose wavelength is strictly controlled is emitted from the light source. This divergent light is converted to parallel light beam by a collimating lens. Subsequently, this parallel light beam is introduced into an optical fiber combination lens, and is condensed onto the focal plane thereof. By placing an optical fiber on this focal plane, the light beam (optical signals) is combined.
At this time, a gradient index lens is used for the collimating lens and the optical fiber combination lens.
Furthermore, recently, the idea has been proposed of using an aspherical lens for the above lenses. The reason for this is to improve the combination efficiency of the light beam with the optical fiber, or of the light beam with the light source. For example, Japanese Patent Application No. 2002-55276 discloses an aspherical lens for use in an optical communication system. The purpose of the aspherical lens disclosed here is to reduce chromatic aberration and obtain a high level of efficiency. Aspherical lenses that may be used in place of gradient index lenses are also available commercially.
The collimating lens of the present invention comprises
a lens having at least one aspherical surface, wherein
the collimating lens is used in a wavelength range of 1.2 xcexcm to 1.7 xcexcm and satisfies the following conditions:
226xe2x89xa6xxe2x89xa6456
yxe2x89xa61.8.
Where x is an infrared analysis indicator defined by x=(n(1.55)xe2x88x921)/(n(1.45)xe2x88x92n(1.65)), y is an index of refraction at a wavelength of 1.55 xcexcm shown by y=n(1.55), and n(a) is the index of refraction at wavelength of a.
It is preferable that said collimating lens satisfies the following condition:
0.6xe2x89xa6xcex1xe2x89xa61.1,
wherein xcex1 is a sagital constant and defined by the formula
xcex1=NA/(n(1.55)xe2x88x921),
wherein NA is an numerical aperture of the collimating lens.
It is preferable that said collimating lens satisfies the following condition:
300xe2x89xa6xxe2x89xa6456.
It is preferable that said collimating lens satisfies the following condition:
1.75xe2x89xa6y 1.8
It is preferable that said collimating lens has an optical surface that has a reflectance of 1% or less in at least a portion of the wavelength range.